This invention relates generally to cyclic nitroxyl compounds useful as anti-oxidant additives for lubricating oil.
Zinc dialkyldithiophosphates (ZDDP) are widely used as lubricant additives to impart anti-wear and anti-oxidant properties. The principal disadvantages of these compounds are that an ash residue is produced by the zinc as the additive is consumed, and that phosphorus is known to affect the efficiency of catalytic converters in motor vehicles, thereby causing emissions problems. Cyclic nitroxyl compounds are disclosed in U.S. Pat. No. 3,532,703 as stabilizers for polyolefins against degradation caused by light. However, this reference does not suggest that such compounds would be useful as additives for lubricating oils, or that a thioamide group would impart useful properties.
The problem addressed by this invention is to find non-metallic oil-soluble anti-oxidant additives for lubricating oils.